In making a reflection hologram, a photosensitive recording element is exposed to two beams of coherent radiation that enter the recording element from opposite sides. One beam, generally referred to as the reference beam, interferes with the other beam, generally referred to as the object beam, which contains the information to be recorded. The intersecting beams produce an interference pattern that is recorded within the photosensitive element by modulating the refractive index throughout the volume. Such holograms are known as volume holograms because the interference pattern is recorded throughout the volume, rather than on the surface, of the recording material.
Reflection holograms may be produced by two well-known methods. In the "on-axis" method a beam of coherent radiation is projected through the photosensitive element onto an object behind the element. The radiation reflected by the object returns and interferes with the incident projected beam in the plane of the recording medium. Reflection holograms also may be produced by an "off-axis" method wherein a reference beam is projected on one side of the photosensitive element and an object beam is projected on the reverse side of the element. Reflection holograms produced by an off-axis process are disclosed in Hartman, U.S. Pat. No. 3,532,406.
Undesired images produced in the recording process are a serious problem in the recording and duplicating of reflection holograms. A portion of the incident beam is reflected at the front and back surfaces of the recording medium. These reflected beams interact with the incident beam to form secondary holograms. Such secondary holograms produce a loss of image quality ("noise"), in the final hologram. Image brightness is also decreased.
Ryan, U.S. Pat. No. 3,833,383, discloses the use of an antihalation layer to prevent the formation of secondary holograms. The antihalation layer is opaque to radiation, however, so only the side of the photosensitive element opposite the antihalation layer can be exposed to radiation. Consequently, this technique is useful only in processes in which both beams are incident on the same side of the photosensitive element, i.e., recording of transmission holograms.
Anti-reflection layers and anti-reflection coated plates, coupled to the photosensitive element with index matching fluids, have been used to reduce noise. However, such layers and plates are expensive and difficult to use. They are wavelength and angle specific so that a different layer or plate is required for each significantly different imaging wavelength and angle. In addition, they provide only a partial solution. Although noise is reduced to some extent, enough remains that quality of the hologram may be affected. Thus, a need exists for a simple, relatively inexpensive process for reducing the noise in recording reflection holograms.